UV mapping , when colouring the outer corners of the canvas , other faces are affected which are not unwrapped
If you unwrap any part of a mesh, a uv map is created. In a UV map all vertices in a mesh needs to have a UV coordinate. Blender gives not yet unwrapped vertices the coordinate U=0.0, V=0.0 (bottom left corner).

Since all your corner faces share the same uv coordinate they will get effected in the same way.

You could use the Face Selection Masking for Painting and give the corner UVs enough space on the UV map to not get effected by the unwrapped faces. Another way would to give the corner faces a different material.